1. Technical Field
The present invention relates to error detection and correction in digital stored media, and more particularly to systems and methods for erasure flagging in errors-and-erasures decoding for enhancing the reliability of storage devices by improving the decoding performance.
2. Description of the Related Art
Error correction codes (ECC) are ubiquitously used in communications and storage devices to improve the reliability of detected bits. Storage devices in particular are prone to errors that occur in bursts in addition to errors that occur at random. Causes of burst errors include mechanical disturbances such as shocks and vibrations (e.g., dropping a device, etc.) and loss of synchronization in the timing recovery loop of the read channel.
It is known that, in the presence of burst errors, the performance of the ECC, and thus of the system, can be significantly improved by resorting to errors-and-erasures decoding (EED) as opposed to error decoding only. Simply speaking, an erasure on a symbol indicates that the detector believes this symbol is not reliable, and thus has a high probability of being detected in error.
A main drawback of EED is that its performance deteriorates quickly when the erasures are not set correctly. There are several approaches that have been taken in the past to solve this problem. In one approach, an erasure is declared (flagged) whenever the symbol amplitude at a threshold detector input lies in a “grey zone” around the detector thresholds. In another, all symbols below a given threshold SNR (signal to noise ratio) are flagged as erasures. In still another, the Euclidean distance of sequences of symbols of a Viterbi detector is used as an erasure indicator. The soft-output Viterbi algorithm has also been used for erasure flagging.
All the above methods have the same drawback. Namely, the erasure flagging method is not sufficiently reliable. The probability of not flagging an erroneous symbol as an erasure (probability of mis-detection) and the probability of declaring an erasure on a correct symbol (false-alarm probability) are both high, thus leading to very small performance gains, or even performance losses with respect to the errors-only decoding case.